The invention is directed to a method for determining the energy content value of electrochemical energy stores in charging and/or discharging mode by measuring the terminal voltage of an energy store. The invention is further directed to an apparatus for the implementation of the method.
Such energy stores, both as individual cells as well as cells arbitrarily interconnected to form a battery, serve in many fashions as traction and starter batteries for land craft, aircraft and water craft, as well as serving for the energy supply of a multitude of portable and stationary systems, of machine controls and device controls, of measuring equipment, signal systems and alarm systems and for solar-electrically operated stores. In many of the said applications, an especially continuous, adequately precise determination and monitoring of the energy content of such energy stores is required in order to avoid an interruption of the operation of the devices operated by an energy store, for the capacity of an energy store can only be fully exploited and an upward transgression or downward transgression of defined charging or discharging limits that is injurious to an energy store can only be avoided given an exact knowledge of the respective energy content. It should thereby be assured that the method can be implemented with simple means and that the apparatus for determining the energy content value can be operated in a surveyable and practical manner.
WO 86/00418 has disclosed a method for monitoring the charging status of chargeable batteries, particularly nickel-cadmium accumulators, whereby a reference value corresponding to a defined charging status is stored, the terminal voltage, the intensity of the current and the time during every charging and discharging cycle of the battery are measured and, taking the polarity of the current into consideration, are converted into a quantity corresponding to the supplied or, respectively, consumed amount of energy from which the actual values of the charging status are calculated proceeding from a reference value, whereby the rate of change of the terminal voltage in the charging and discharging cycle is monitored, whereby at least one point in time where at a characteristic, more pronounced rate of voltage change occurs is determined after traversing a phase of relatively uniform and low rate of voltage change, and whereby the actual value of the charging status that is calculated at this point in time or at a point in time delayed in defined fashion is balanced to the prescribed reference value.
For the implementation of this method, a means is provided therein comprising arrangements for measuring the terminal voltage of the battery, the direction and amplitude of the charging or, respectively, discharging current and the battery temperature, a program-controlled arithmetic unit that is connected to the measuring arrangements and controlled such that it monitors the rate of change of the terminal voltage during all charging and discharging cycles of the battery and determines an actual value of the charging status when, following an essentially constant phase, the rate of voltage change experiences a more pronounced change, a memory that is connected to the arithmetic unit for storing the reference values and the actual values developed in the arithmetic unit for the supplied or, respectively, used amounts of energy, and an interface and driver circuit for coupling the arithmetic unit to peripheral input and output devices.
This method is affected by the considerable disadvantage that, among other things, a measurement of the charging current or, respectively, discharging current is required therein for calculating the charge status and that a precision resistor must be connected into the circuit of the battery for this purpose, the required reference value in this respect being calculated at this precision resistor. Further, the considerable metrological outlay required is disadvantageous, this making an economical employment thereof seem meaningful only in appropriate large-scale systems.
A method disclosed by German Published Application 34 16 849 comprising an arrangement for measuring the charge status of batteries also uses an initialized ampere-hour meter for the proximation of the charge status of the battery system to be measured, this proceeding on the basis of a completely charged battery, whereby the output signal of the meter follows the true charge status of the battery system closely enough.
This method is likewise affected by the disadvantage of an operation performed on the circuit of the battery system, particularly since the current determined therein at a measuring shunt adequately allows a calculation together with other reaction values only given an adequately constant loading of the battery.
In the method disclosed by German Published Application 29 52 835, moreover, an acquisition of the current flux through the accumulator is required for the determination of the capacity of an accumulator, so that an operation performed on the circuit is also required here, this being affected by the disadvantages already set forth.